Ultra-capacitors are uniquely suited for use in regenerative braking because they are able to store electrical charge at a rapid rate. In an electrical system of an automotive vehicle having an ultra-capacitor and an alternator, regardless of alternator voltage level used for charging, the vehicle electrical system must regulate voltage to a minimum system voltage to maintain the functionality of electrical components in the vehicle. This minimum system voltage limits the amount of charge that can be utilized from the ultra-capacitor, since the ultra-capacitor can never be fully discharged below this minimum system voltage. Because of this, a significant portion of the charge stored in the ultra-capacitor is typically never utilized to provide power to electrical components in the vehicle electrical system.
For example, in an automotive vehicle having a 24 VDC (nominal) charging system with a 12 VDC (nominal) vehicle electrical system architecture, the ultra-capacitor is charged to about 25 VDC and can only be depleted until the voltage reaches a nominal voltage depletion level, in this case, about 14.8 VDC. VDC is volts direct current. In a vehicle having a 12 VDC (nominal) charging system with a 12 VDC (nominal) vehicle electrical system architecture, the ultra-capacitor is charged to about 15 VDC and can only be depleted until the voltage reaches approximately about 12.6 VDC. This limits the amount of utility for an ultra-capacitor of a given size as the remaining charge of the ultra-capacitor is not able to be released. Therefore, when the next regenerative braking event occurs, only some of the total capacity of the ultra-capacitor can be utilized to store the energy from the regenerative braking.